1. Field of the Invention
This invention relates to decreasing the anisotropy of nonwoven materials and particularly spunlaced nonwovens.
2. Description of Related Art
In the manufacture of nonwoven fabrics, it is common to experience anisotropic properties. Probably the most important property is tensile strength of the fabric, wherein the strength in the “machine-direction” (MD) is notably higher than that in the “cross-machine-direction” (XD). This MD/XD ratio of strength, typically greater than unity, is a disadvantage versus other fabrics such as woven goods where the strengths are relatively balanced. In nonwovens, this MD/XD ratio is often at least 2:1. It is often higher in the case of fabrics made from carded web substrates where the ratio can approach 4:1 or even 5:1. Even spunbonded fabrics exhibit this same imbalance of properties, which is exacerbated by high laydown speeds.
Attempts to control or reduce this ratio by conventional means include cross-lapping of air-laid or carded webs, stretching a formed fabric in the XD direction, or the use of a “scrambler” roll after a card doffer roll. In the case of spunbonded fabrics, such as Typar®, available from E.I. du Pont de Nemours and Company, Wilmington, Del. (hereafter DuPont), curtains of fibers are oscillated with rotary air jets in both the MD or XD direction. To achieve balanced properties, fibers must be oriented in the direction of the desired strength. The relatively small number of fibers in the cross machine direction in contrast to the larger number in the machine direction corresponds to the relatively lower XD strength.